Plasma Separation Membrane

ABSTRACT

A process for manufacturing of an asymmetric hollow fibre membrane, comprising the steps of extruding a polymer solution through the outer ring slit of a hollow fibre spinning nozzle, simultaneously extruding a centre fluid through the inner bore of the hollow fibre spinning nozzle, into a precipitation bath, whereby the polymer solution contains 10 to 26 wt-% of polysulfone (PSU), polyethersulfone (PES) or polyarylethersulfone (PAES), 8 to 15 wt-% polyvinylpyrrolidone (PVP) and 60 to 80 wt-% N-alkyl-2-pyrrolidone (NAP), the centre fluid contains 60 to 70 wt-% N-alkyl-2-pyrrolidone (NAP) and 30 to 40 wt-% water, and the precipitation bath contains 70 to 82 wt-% N-alkyl-2-pyrrolidone (NAP) and 18 to 30 wt-% water.

BACKGROUND OF THE INVENTION

The present invention is directed to a process for manufacturing of anasymmetric hollow fibre membrane, which is, among other applications,particularly suitable for plasma separation, but which can alsoadvantageously be used in certain technical applications. Furthermorethis invention is directed to such membranes being producible by theprocess of the invention, and to the use of such membranes for plasmaseparation, plasma filtration, micro filtration, plasma therapy or cellfiltration applications.

Plasma separation or apheresis is a medical technology in which theblood of a donor or patient is separated into the plasma, i.e. the cellfree component in blood, and the blood cells. Plasma separation may beconducted for several reasons.

In the therapeutical plasmapheresis the separated plasma of a patient'sblood is discarded and replaced by a substitute solution or by donorplasma, and is reinfused into the patient. This approach is useful inthe treatment of several diseases and disorders. For example, inimmunological diseases the plasmapheresis is useful to exchangeantibodies, antigens, immune complexes or immune globulins. Innon-immunological diseases the plasmapheresis allows for the depletionof metabolites, degradation products, as well as endogenous andexogenous toxins.

In a variant of therapeutical plasmapheresis, plasma fractionation, theseparated plasma of a patient's blood undergoes a second stage offurther separation into high molecular and low molecular plasmafractions. The high molecular fraction is discarded, and the lowmolecular fraction of the plasma and the cellular components of theblood are reinfused into the patient.

In an application, called plasma donation, the separated blood plasmafrom healthy donors is used for therapeutical plasma exchange, or forthe isolation of plasma components for pharmaceutical purposes.

The separation of whole blood into plasma and cellular components can beachieved either by centrifugation or by passing the blood through aplasma separation membrane. During the development of plasmapheresis,discontinuous centrifuges have been used first, which have then, at thebeginning of the 70s, been replaced by continuous centrifugationsystems. Centrifugation techniques have the advantage of being fast andcost effective, however, they often suffer from leaving impurities ofcells or cell debries in the separated plasma. At the end of the 70s,the first membrane systems have been introduced for the plasmapheresisto overcome the disadvantages of centrifugation systems.

While being related to it, the requirements of plasma separationmembranes are quite distinct from the requirements of dialysismembranes. Plasma separation uses the effect of separation byfiltration, whereas dialysis rather uses osmosis and diffusion.

Some of the essential design criteria of a plasma separation membraneare the wall-shear rate, the transmembrane pressure drop and the plasmafiltration rate.

The wall-shear rate in a hollow fibre membrane system is calculated bythe following equation:

$\gamma_{w} = \frac{4\; Q_{B}}{N\; \pi \; r^{3}}$

wherein N is the number of hollow fibres, having the inner radius r, towhich blood flow Q_(B) is distributed. By the decrease of the plasmaportion the blood flow changes across the length of the hollow fibre.This must be considered in the calculation of the wall-shear rate.

The transmembrane pressure (TMP) is another important parameter which isdefined as the difference in pressure between the two sides of themembrane. The transmembrane pressure is the driving force for themembrane separation. In general, an increase in the transmembranepressure increases the flux across the membrane. The exception to thisgeneralization occurs if a compressible filter cake is present on thesurface of the membrane. The transmembrane pressure is calculated by thefollowing equation:

${TMP} = {\frac{P_{Bi} + P_{Bo}}{2} - P_{F}}$

wherein P_(Bi) is the pressure at the blood entrance, P_(Bo) is thepressure at the blood exit, and P_(F) is the pressure on the filtrateside of the membrane (plasma side).

The sieving coefficient determines how much of a compound will beeliminated by a filtration process. The sieving coefficient is definedas the ratio of the concentration of a compound in the filtrate to theconcentration of this compound in the blood. A sieving coefficient of“0” means that the compound can not pass the membrane. A sievingcoefficient of “1” means that 100% of the compound can pass themembrane. For the design of plasma separation membranes it is desiredthat the whole spectrum of plasma proteins can pass the filtrationmembrane whereas the cellular components are completely retained.

The requirements of a plasma separation membrane for plasmapheresis canbe summarized as by the following characteristics:

-   -   high permeability or high sieving coefficient for the whole        spectrum of plasma proteins and lipoproteins;    -   high surface porosity and total porosity of the membrane to        achieve high filtration performance;    -   a hydrophilic, spontaneously wettable membrane structure;    -   low fouling properties for long term stable filtration;    -   low protein adsorption;    -   smooth surfaces in contact with blood;    -   low or no tendency to haemolysis during blood processing;    -   constant sieving properties and filtration behaviour over the        whole treatment period;    -   high biocompatibility, no complement activation, low        thrombogenicity; mechanical stability;    -   sterilizability by steam, gamma radiation and/or ETO;    -   low amount of extractables.

DESCRIPTION OF THE INVENTION

The object of the present invention was to provide a novel hollow fibremembrane, particularly useful in plasma separation applications, havingimproved properties over the prior art membranes, especially in respectof the above-mentioned characteristics, and a process of producing sucha membrane.

This and other objects are solved by a membrane being obtainable orobtained by the process of the present invention. Thus, according to thepresent invention there is provided

a process for manufacturing of an asymmetric hollow fibre membrane,comprising the steps of

-   -   extruding a polymer solution through the outer ring slit of a        hollow fibre spinning nozzle, simultaneously extruding a centre        fluid through the inner bore of the hollow fibre spinning        nozzle, into a precipitation bath, whereby    -   the polymer solution contains 10 to 26 wt-% of polysulfone        (PSU), polyethersulfone (PES) or polyarylethersulfone (PAES), 8        to 15 wt-% polyvinylpyrrolidone (PVP) and 60 to 80 wt-%        N-alkyl-2-pyrrolidone (NAP),    -   the centre fluid contains 60 to 70 wt-% N-alkyl-2-pyrrolidone        (NAP) and 30 to 40 wt-% water, and    -   the precipitation bath contains 70 to 82 wt-%        N-alkyl-2-pyrrolidone (NAP) and 18 to 30 wt-% water.

Even though some of the prior art membranes may, in comparison to themembrane produced according to the present invention, exhibit equal orsimilar characteristics in respect of one or several properties, theasymmetric hollow fibre membrane produced according to the presentinvention is superior in the combination of properties desired for aseparation membrane, particularly a plasma separation membrane forplasmapheresis.

The asymmetric hollow fibre membrane produced according to the presentinvention exhibits high permeability for the whole spectrum of plasmaproteins and lipoproteins, reflected by a high sieving coefficient.Preferably the sieving coefficient of the asymmetric hollow fibremembrane of the invention for all plasma proteins is >0.90, morepreferably is >0.95.

The asymmetric hollow fibre membrane produced according to the presentinvention exhibits a high surface porosity and total porosity of themembrane to achieve high filtration performance. It further has ahydrophilic, spontaneously wettable membrane structure, low foulingproperties for long term stable filtration, and low protein adsorption.The asymmetric hollow fibre membrane produced according to the presentinvention further has smooth surfaces in contact with blood which avoidsor minimizes haemolysis during blood processing. The membrane showsconstant sieving properties and filtration behaviour over the wholetreatment period. It further exhibits high biocompatibility, nocomplement activation and low thrombogenicity. The mechanical stabilityof the membrane is excellent, and it is sterilizable by steam, gammaradiation and/or ETO.

In the process of the present invention it is required that the polymersolution contains 10 to 26 wt-% of polysulfone (PSU), polyethersulfone(PES) or polyarylethersulfone (PAES), whereby the usepolyarylethersulfone (PAES) is most preferred. The polymer solutionfurther contains 8 to 15 wt-% polyvinylpyrrolidone (PVP) and 60 to 80wt-% N-alkyl-2-pyrrolidone (NAP).

Using in the polymer solution less than 10 wt-% of polysulfone (PSU),polyethersulfone (PES) or polyarylethersulfone (PAES) causes themembrane to become very brittle compared to the membrane according tothe present invention. At the same time the combination of desiredmembrane properties can not be achieved any longer. And, using more than26 wt-% of polysulfone (PSU), polyethersulfone (PES) orpolyarylethersulfone (PAES) this results in difficulties to prepare thepolymer solution and to perform the spinning of hollow fibre membranesbecause of a too high viscosity of the polymer solution.

Using in the polymer solution less than 8 wt-% of polyvinylpyrrolidone(PVP) does not result in the required hydrophilicity (spontaneouslywettable morphology) and the desired overall structure of the membrane.And, using more than 15 wt-% of polyvinylpyrrolidone (PVP) causes anextremely high viscosity of the polymer solution and complicatesspinning of the hollow fibre membrane. At the same time the amount ofextractables (PVP) increases very much. In addition to this, too highamounts of PVP lower the mechanical properties.

Using in the polymer solution less than 60 wt-% of N-alkyl-2-pyrrolidone(NAP) causes difficulties to process the polymer solution to form amembrane, due to an extremely high solution viscosity. And, using morethan 80 wt-% N-alkyl-2-pyrrolidone (NAP) results in low solutionviscosity. The polymer present in such a solution will not provide anideal microporous membrane for plasma separation purposes.

In one embodiment of the process of the present invention the polymersolution contains 15 to 21 wt-% of polysulfone (PSU), polyethersulfone(PES) or polyarylethersulfone (PAES), 10 to 12.5 wt-%polyvinylpyrrolidone (PVP) and 66 to 76 wt-% N-alkyl-2-pyrrolidone(NAP).

In another embodiment of the process of the present invention thepolymer solution contains 17 to 19 wt-% of polysulfone (PSU),polyethersulfone (PES) or polyarylethersulfone (PAES), 10.75 to 11.75wt-% polyvinylpyrrolidone (PVP) and 69 to 72.5 wt-%N-alkyl-2-pyrrolidone (NAP).

In the process of the present invention it is required that the centrefluid contains 60 to 70 wt-% N-alkyl-2-pyrrolidone (NAP) and 30 to 40wt-% water.

Using in the centre fluid less that 60 wt-% N-alkyl-2-pyrrolidone (NAP)causes the membrane to become too tight, i.e. the selective pore size ofthe membrane becomes too small to allow the majority of the plasmaproteins to pass the membrane structure. And, using more than 70 wt-%N-alkyl-2-pyrrolidone (NAP) causes the membrane to get a rough surfacecausing haemolysis during blood treatment.

In one embodiment of the process of the present invention the centrefluid contains 61 to 67 wt-% N-alkyl-2-pyrrolidone (NAP) and 33 to 39wt-% water.

In another embodiment of the process of the present invention the centrefluid contains 63 to 65 wt-% N-alkyl-2-pyrrolidone (NAP) and 35 to 37wt-% water.

In the process of the present invention it is required that theprecipitation bath contains 70 to 82 wt-% N-alkyl-2-pyrrolidone (NAP)and 18 to 30 wt-% water.

Using in the precipitation bath less that 70 wt-% N-alkyl-2-pyrrolidone(NAP) causes the membrane to become too tight on the outside and/or theoverall structure of the membrane. This results is drastically reducedtotal plasma protein sieving coefficients. And, using more than 82 wt-%N-alkyl-2-pyrrolidone (NAP) causes the membrane to become instableduring the membrane formation procedure.

In one embodiment of the process of the present invention theprecipitation bath contains 73 to 79 wt-% N-alkyl-2-pyrrolidone (NAP)and 21 to 27 wt-% water.

In another embodiment of the process of the present invention theprecipitation bath contains 75 to 77 wt-% N-alkyl-2-pyrrolidone (NAP)and 23 to 25 wt-% water.

In the process of the present invention the N-alkyl-2-pyrrolidone (NAP)in the polymer solution, in the centre fluid and in the precipitationbath can be the same or different, however most preferably is the samein all three solutions.

Preferably the N-alkyl-2-pyrrolidone (NAP) is selected from the groupconsisting of N-methyl-2-pyrrolidone (NMP), N-ethyl-2-pyrrolidone (NEP),N-octyl-2-pyrrolidone (NOP) or mixtures thereof, wherebyN-methyl-2-pyrrolidone (NMP) is most preferred.

In another embodiment of the process of present invention thepolyvinylpyrrolidone (PVP) in the polymer solution consists of a blendof at least two homo-polymers of polyvinylpyrrolidone whereby one of thehomo-polymers of polyvinylpyrrolidone (=low molecular weight PVP) havingan average relative molecular weight of about 10,000 g/mole to 100,000g/mole, preferably about 30,000 g/mole to 60,000 g/mole, and another oneof the homo-polymers of polyvinylpyrrolidone (=high molecular weightPVP) having an average relative molecular weight of about 500,000 g/moleto 2,000,000 g/mole, preferably about 800,000 g/mole to 2,000,000g/mole. It is even more preferred if the polyvinylpyrrolidone (PVP) inthe polymer solution consists of a blend of only two homo-polymers ofpolyvinylpyrrolidone of the afore-mentioned type.

Using a blend of two homo-polymers of polyvinylpyrrolidone of differentaverage relative molecular weights results in a desired hydrophilicity,structure and morphology of the membrane. Without being bound by theory,it is assumed that during the production process the high molecularweight PVP remains incorporated in the hollow fibre membrane, whereasthe majority of the low molecular weight PVP is washed out.

In one embodiment of the invention the low molecular weight PVP in thepolymer solution is present in an amount of 5.7 to 11.7 wt-% and thehigh molecular weight PVP is present in an amount of 2.3 to 4.3 wt-%,based on the total weight of the polymer solution. In another embodimentthe low molecular weight PVP is present in an amount of 7.1 to 8.9 wt-%and the high molecular weight PVP is present in an amount of 2.9 to 3.6wt-%, based on the total weight of the polymer solution. In a furtherembodiment the low molecular weight PVP is present in an amount of about3.25 wt-% and the high molecular weight PVP is present in an amount ofabout 8.0 wt-%, based on the total weight of the polymer solution. Thetotal amount of PVP should, however, be within the ranges indicatedabove. If the concentration of high molecular weight PVP is too low,then the degree of hydrophilicity of the membrane might not besufficient. If the concentration of high molecular weight PVP is toohigh, then the viscosity of the polymer solution might be too highcausing serious processability problems. If the concentration of lowmolecular weight PVP is to low, then this results in a closed cellstructure instead of a open membrane structure. If the concentration oflow molecular weight PVP is to high, then this would require the removalof the low molecular weight PVP by exhaustive washing. If too much ofthe low molecular weight PVP remains in the membrane product themembrane could not be used for blood treatment because the extractablePVP would contaminate the blood or plasma.

In another embodiment of the process of the present invention theprecipitation bath has a temperature in the range 10 to 60° C.,preferably 20 to 50° C., more preferably 30 to 40° C. If the temperatureof the precipitation bath is too low the precipitation of the membranemight be too slow, which could result in a too dense structure on theoutside. If the temperature of the precipitation bath is too high thefibre becomes instable during the precipitation procedure.

In another embodiment of the process of the present invention the hollowfibre spinning nozzle (die; spinneret) is held at a temperature in therange 40 to 75° C., preferably 45 to 70° C., more preferably 50 to 65°C., most preferably at about 50° C. If the temperature of the hollowfibre spinning nozzle is too low the pressure drop in the spinning dieis increasing. The pressure drop increases exponentially if dietemperature is lowered. A high pressure drop results in unstablespinning conditions, i.e. rougher outer surface, increased variations indimension etc. If the temperature of the hollow fibre spinning nozzle istoo high the speed of polymer outflow out of the die might be too fast.This would result in unstable spinning conditions.

In another embodiment of the process of the present invention thedistance (gap) between the discharge outlet of the hollow fibre spinningnozzle (die; spinneret) to the surface of the precipitation bath is inthe range of 0.5 to 10 cm, preferably 1 to 8 cm, more preferably 2 to 5cm. If the distance between the discharge outlet of the hollow fibrespinning nozzle to the surface of the precipitation bath is too low thedesired product properties will not be achieved, e.g. smooth innersurface. If the distance between the discharge outlet of the hollowfibre spinning nozzle to the surface of the precipitation bath is toohigh the spinning becomes difficult or even impossible. The stability ofthe fibre is not provided if the distance in increased above the givenlimit.

In another embodiment of the process of the present invention thespinning speed of the hollow fibre membrane is in the range of 1 to 20m/min, preferably 3 to 15 m/min, more preferably 5 to 10 m/min. If thespinning speed of the hollow fibre membrane is too low the spinningconditions become unstable and the desired membrane dimensions cannot beachieved. If the spinning speed of the hollow fibre membrane is too highthe residence time in the precipitation bath is decreasing, whichresults in extremely dense layers in the cross section. These denselayers do not allow a sufficient high sieving coefficient for all plasmaproteins.

In another embodiment of the process of the present invention thepolymer solution has a viscosity, measured at room temperature, of30.000 to 100.000 mPa×s (Centipoise). If the viscosity is lower than30.000 mPa×s (Centipoise) then the stability of the fiber in theprecipitation bath is not provided, which results in fiber breakingduring the spinning process. If the viscosity is higher than 100.000mPa×s (Centipoise) then solution handling, i.e. solution preparation andpumping of the solution becomes difficult, and the pressure drop in thespinning die becomes too high.

The present invention covers also hollow fibre membranes obtainable orobtained by the process of the invention.

In one embodiment of the present invention the hollow fibre membrane ischaracterized by a total plasma protein sieving coefficient of >0.90,preferably >0.95. A high sieving coefficient for total plasma protein isessential to the membrane if it is used for example as a plasmaseparation membrane. In plasma separation it is desired to have thetotal plasma protein in the separated plasma fraction, whereas thelarger corpuscular components of the blood, like blood cells and celldebris, are retained by the membrane.

For plasma separation applications it is preferred that the hollow fibremembrane shall have an inner diameter in the range of 100 to 500 μm,preferably 150 to 450 μm, more preferably 200 to 400 μm. Lower innerdiameters are disadvantageous because they result in too high wall shearrates and increased pressure drop in the fibre or in the wholefiltration module. On the other hand, if the inner diameters are toohigh, this would result in too low shear rates which increase the riskof haemolysis at low transmembrane pressures (TMP).

It is further preferred for plasma separation applications that thehollow fibre membrane shall have a wall thickness in the range of 20 to150 μm, preferably 30 to 125 μm, more preferably 40 to 100 μm. Lowerwall thicknesses are disadvantageous due to reduced mechanicalproperties of the fibre during production and during its use in theplasma separation module itself. Higher wall thicknesses aredisadvantageous because they require increased time intervals to performthe phase inversion process resulting in instable process conditions andan instable membrane.

It is further preferred for plasma separation applications that thehollow fibre membrane shall have an average pore diameter of theselective separation layer in the membrane in the range of 0.1 to 1 μm,preferably 0.1 to 0.7 μm, more preferably 0.1 to 0.4 μm. Lower averagepore diameters of the selective separation layer are disadvantageous dueto incomplete passage of total plasma proteins through the porousstructure. Higher average pore diameters of the selective separationlayer are disadvantageous due to an increased risk of haemolysis (cellrupture).

In another embodiment of the present invention the hollow fibre membraneis characterized by a pore size distribution wherein the pore sizes atthe inner wall surface of the membrane (lumen surface) are smallest andincreasing towards the outer wall surface of the membrane where the poresizes are biggest. This structure has the advantage that cellularfractions cannot enter the porous substructure, which could result inblocking and even rupture of some cells. At the same time the smoothsurface having the smallest pores at the blood contacting surfaceresults in outstanding biocompatibility/hemocompatibility, i.e. noactivation of cellular and non-cellular fractions of the plasma.

The process of the present invention for the manufacturing ofmicroporous membranes, particularly plasma separation membranes, is adiffusion induced phase separation (DIPS) procedure. The averagediameter of the selective pores of such plasma separation membranes isin the range of 0.1 to 0.4 μm. The porous structure next to the“selective” pore size region has larger pores up to several μm. Toachieve such larger pores next to the selective layer of the structurethe phase separation process has to be performed slowly to allowgeneration of the pores beginning with a pore size of approximately 0.1μm and larger. To allow a slow phase separation process the amount ofsolvent for the polymer, in our case NAP (N-alkyl-2-pyrrolidone),preferably NMP (N-methyl-2-pyrrolidone), has to be sufficient high.However, high concentration of solvent in the centre fluid (in the boreof the hollow fiber) and the precipitation bath creates instability ofthe fibre. This makes it difficult to get stable fibers into theprecipitation bath and out of this bath. The challenge of the presentinvention was to adjust sufficient high NAP concentration during theprecipitation procedure (in the centre and the precipitation bath) andat the same time allow processability of the fiber.

One of the major findings of the present invention was that increasingthe amount of NAP in the centre fluid leads to an increase in pore sizeat the inner (selective) layer. Pore size is of course important toachieve the required plasma protein passage (high sieving coefficient).The width of the pore size distribution is also of major importance toallow all plasma proteins to pass this membrane. However, increasing theNAP concentration in the centre fluid leads to a slower phase separationprocedure (slower precipitation and slower membrane structureformation), which results in decreased stability of the membrane.Further, at the same time the NAP concentration in the centre fluid isincreased above a certain value, also the roughness of the innermembrane surface is increased, which again results in increasedhaemolysis. The challenge was to find a production window that allows toadjust (i) sufficient high concentration of NAP in the centre fluid togenerate a morphology that allows all plasma proteins to pass, (ii)acceptable roughness to have no or reduced haemolysis, (iii) a NAPconcentration in the precipitation bath to get a sufficient openstructure on the outside and in the cross section, (iv) stable spinningconditions.

The present inventors have now identified a process route allowing theproduction of plasma separation membranes fulfilling the desiredproperty profile.

An example of preferred process conditions for the production of aplasma separation membrane according to the present invention isdisplayed in Table 1 (see also example 7 below). The polymer solution ispumped through a spinning die and the liquid hollow fibre is formed. TheNMP concentration in the centre fluid leads to a microporous openstructure at the inner side of the membrane. The smallest pores aredirectly at the lumen side. The increased concentration of NMP in theprecipitation bath leads to a very open outside and overall (crosssection) structure. The overall structure and the pores at the outsideof the membrane are much bigger (see FIG. 1) The selective layer is inthis case in direct blood contact. The challenge of the invention was toadjust the spinning conditions to fulfil the profile of the membrane,i.e. biocompatibility, haemolysis and high sieving coefficient and highfiltration rate over time.

TABLE 1 Conditions for the production of a plasma separation membrane ofthe present invention. Composition of the polymer solution [wt-%] PAES:18% PVP (high mol. weight): 3.25% PVP (low mol. weight): 8% NMP: 70.75%Composition of the centre fluid [wt-%] H₂O: 36% NMP: 64% Composition ofthe precipitation bath [wt-%] NMP: 78% Temperature of the precipitationbath [° C.] 30° C. Distance between die and precipitation bath [cm] 3 cmTemperature of the die [° C.] 60° C. Spinning speed [m/min] 5 m/minViscosity [mPa × s] 58.900 mPa × s

Materials and Methods Viscosity Measurement

The term “viscosity” in respect of the polymer solution of the presentinvention means the dynamic viscosity, if not otherwise indicated. TheUnit of the dynamic viscosity of the polymer solution is given inCentipoise (cp) or mPa×s. To measure the viscosity of the polymersolution a commercial Rheometer from Rheometric Scientific Ltd. (SR2000) was used. The polymer solution is placed between twotemperature-controlled plates. The measurement is performed at 22° C.All other measurement condition are according to the manufacturer'sinstructions.

Membrane Bundle Preparation a) Preparation of Hand Bundles:

The preparation of membrane bundles after the spinning process isnecessary to prepare the fibre bundle in an adequate way for theperformance tests (measurement of the total protein sieving coefficientand determination of the haemolysis properties of the membrane). Thefirst process step is to fix the fibres near their ends by a filament.The fibre bundle is transferred into a sterilization tube and thensterilized. After the sterilization, the fibre bundle is cut to adefined length of 23 cm. The next process step consists of closing theends of the fibres. An optical control ensures that all fibres are wellclosed. Then, the ends of the fibre bundle are transferred into apotting cap. The potting cap is fixed mechanically, and a potting tubeis put over the potting caps. Afterwards, the potting is done withpolyurethane. After the potting, it has to be ensured that thepolyurethane can harden for at least one day. In the next process step,the potted membrane bundle is cut to a defined length. The last processstep consists of an optic control of the fibre bundle. During thisprocess step, the quality of the cut (is the cut smooth or are there anydamages of the knife) and the quality of the potting (is the number ofopen fibres of the spinning process reduced by fibres that are potted orare there any visible voids where the there is no polyurethane) arecontrolled. After the optical control, the membrane bundles are storeddry before they are used for the different performance tests.

b) Preparation of Minimodules:

Minimodules, i.e. fibre bundles in a housing, are prepared by similarprocess steps as in the preparation of hand bundles. The minimodules areneeded to ensure a protection of the fibres and a very cleanmanufacturing method as the biocompatibility tests are carried out withhuman plasma. The manufacturing of the minimodules differs in thefollowing points over the preparation of hand bundles in that i) thefibre bundle is cut to a defined length of 20 cm, ii) the fibre bundleis transferred into the housing before the fibres are closed, and iii)the minimodule is put into a vacuum drying oven over night before thepotting process.

Total Protein Sieving Coefficient

The total protein sieving coefficient of a membrane is determined bypumping bovine blood with a defined haematocrit under defined conditions(shear rate [by adjusting the Q_(B)], TMP) through a membrane bundle anddetermining the concentration of the total protein in the feed, in theretentate and in the filtrate. If the concentration of the total proteinin the filtrate is zero, a sieving coefficient of 0% is obtained. If theconcentration of the total protein in the filtrate equals theconcentration of the protein in the feed and the retentate, a sievingcoefficient of 100% is obtained. The sampling takes place at theearliest 10 minutes after a constant TMP is adjusted. The test iscarried out in the single-pass modus. The bovine blood is heated up by aheat exchanger to 37° C. before entering the fibre bundle. The retentateand the feed samples are centrifuged before the determination of theconcentration of the total protein. The determination of the totalprotein is done photometric. The test can be modified to determine thelong-term stability of the total protein sieving coefficient. In thiscase, a constant TMP is applied over a longer time schedule.

Haemolysis Test

The haemolysis test is carried out in a similar way as the sievingcoefficient test described before. The applied transmembrane pressuresare in the range of 30 to 150 mmHg. Before the sampling, at least 10minutes are waited to ensure an equilibrated situation. After the test,the pool samples are centrifuged; no retentate samples are taken for thedetermination of the free haemoglobin. The determination of the freehaemoglobin is done photometric. The value of the free haemoglobin inthe filtrate is adjusted with the value in the pool sample to receivethe content of free haemoglobin generated by the membrane. In parallel,a standard curve is created to get the correlation between the measuredoptical density to the content of free haemoglobin. The standard curveis prepared by diluting one ml of bovine blood directly at the beginningwith 9 ml of distilled water. After centrifugation, 1 ml of thesupernatant is taken and is diluted with 9 ml of isotonic sodiumchloride solution. This represents the 1% standard. Starting with this1% standard a series of further concentrations in the range of 0.05 to1% are produced by dilution. Using these concentrations the standardcurve is created by measuring the corresponding optical density. A levelof haemoglobin below 0.2 in the generated plasma fraction ischaracterized as “non” or “low” haemolytic. Concentrations above 0.2 canbe identified visually (colour change) as haemolytic. Detailedmeasurements are performed photometrically.

Biocompatibility Testing

The following two methods are used to characterize the biocompatibilityproperties of the membrane:

a) Thrombogenicity:

Thrombin-Antithrombin III (TAT) levels are measured and platelet countsare done after passage of platelet rich plasma (PRP) along the membrane,through the membrane and in the pool as a marker for thrombogenicity.The experiment is carried out in a recirculating modus as a high volumeof plasma is required to test in the “single pass modus”.

b) Complement Activation:

Complement activation, as generated by the terminal complement complex(TCC), is measured before and after the passage of fresh human plasmathrough the minimodule. Additionally, the generation of TCC in thefiltrate is measured. The experiment is carried out in a recirculatingmodus, since a high volume of plasma is required to test in the “singlepass modus”. The details of the complement activation measurement are asdescribed by Deppisch, R., et al., Fluid Phase Generation of TerminalComplement Complex as a Novel Index of Biocompatibility. KidneyInternational, 1990. 37: p. 696-706.

Complement activation is not only related to cellular activation butalso to the activation of the plasmatic fraction. In the case of plasmaseparation and subsequent treatment, for example adsorption, doublefiltration complement activation becomes a major issue. In case ofincreased complement activation, i.e. generation of TCC, the activatedplasma may cause severe health problems to a patient, if it isre-infused into a patient.

EXAMPLES Example 1

A polymer solution was prepared by dissolving 18.0 wt-% polyethersulfone(PES; BASF Ultrason 6020), 3.25 wt-% low molecular weightpolyvinylpyrrolidone (PVP; BASF K30) and 8.0 wt-% high molecular weightpolyvinylpyrrolidone (PVP; BASF K85 or K90) in 70.75 wt-%N-Methylpyrrolidone (NMP). The viscosity of the polymer solution at roomtemperature was 64250 mPa×s.

To prepare the solution, NMP was placed in a three neck-flask withfinger-paddle agitator in the centre neck. Then, the PVP was added tothe NMP and stirred at 50° C. until a homogeneous clear solution isformed. Finally, the polyethersulfone (PES) was added. The mixture wasstirred at 50° C. until a clear high viscous solution is obtained. Thewarm solution was cooled down to 20° C. and degassed. To fully degas thesolution the highly viscous polymer solution was transferred into astable stainless steel container, the container was closed tightly andvacuum was applied to the container. The solution was degassed at 50mmHg for 6 hours. During this degassing procedure the container wasmoved to create a larger surface and thinner film thickness of thepolymer solution in the container to improve the degassing procedure.

To form a membrane the polymer solution was heated up to 50° C. andpassed through a spinning die into a precipitation bath. As centre fluida mixture of 38.50 wt-% water and 61.50 wt.-% NMP was used. Thetemperature of the die was 50° C. The hollow fibre membrane was formedat a spinning speed of 5 m/min. The liquid capillary leaving the die waspassed into a precipitation bath consisting of 81 wt-% NMP and 19 wt-%water and having a temperature of 40° C. The distance between the exitof the die and the precipitation bath was 4 cm. The formed hollow fibremembrane was guided through 5 different water baths having a temperatureof 65° C.

The resulting hollow fibre membrane had an inner diameter of 263 μm, anouter diameter of 347 μm and a fully asymmetric membrane structure. Themeasured total protein sieving coefficient was 95% at a transmembranepressure (TMP) of 110 mmHg (Mean Blood flow Q_(B): 2.3 ml/min, meanshear rate: 200 1/s). The degree of free haemoglobin as the correctedfiltrate value (see description of methods) was below the border ofstarting haemolysis of 0.2 for the tested value of 110 mmHg.

Scanning electron micrographs of the inner surface and the cross sectionof the membrane are shown in FIG. 2. The membrane wall shows anasymmetric structure having an overall sponge like structure. The outersurface shows a very large pore size.

Example 2

In Example 2 the same compositions of the polymer solution and thecentre fluid were used as in Example 1. The viscosity of the polymersolution at room temperature was 64250 mPa×s. The precipitation bathconsisted of 82 wt-% NMP and 18 wt-% water. The membrane formationprocedure was the same as in Example 1 with the exceptions that thedistance between the die and the precipitation bath was 0.5 cm, and thespinning velocity was 10 m/min.

The resulting hollow fibre membrane had an inner diameter of 257 μm, anouter diameter of 337 μm and a fully asymmetric membrane structure. Thetotal protein sieving coefficient was 97% at a transmembrane pressure(TMP) of 30 and 70 mmHg and 96% at a transmembrane pressure (TMP) of 110mmHg (Mean Blood flow Q_(B): 2.3 ml/min, mean shear rate: 190 1/s). Thedegree of free haemoglobin as the corrected filtrate value was below theborder of starting haemolysis of 0.2 for the tested values of 30, 70 and110 mmHg.

Example 3

In Example 3 the same compositions of the polymer solution and thecentre fluid were used as in Example 1. The viscosity of the polymersolution at room temperature was 64250 mPa×s. The precipitation bathconsisted of 82 wt-% NMP and 18 wt-% water. The membrane formationprocedure was the same as in Example 1 with the exceptions that thedistance between the die and the precipitation bath was 10 cm, and thespinning velocity was 10 m/min.

The resulting hollow fibre membrane had an inner diameter of 257 μm, anouter diameter of 335 μm and a fully asymmetric membrane structure. Thetotal protein sieving coefficient was 98% at a transmembrane pressure(TMP) of 30 mmHg (Mean Blood flow Q_(B): 2.5 ml/min, mean shear rate:200 1/s). The degree of free haemoglobin as the corrected filtrate value(compare description of the method) was below the border of startinghaemolysis of 0.2 for the tested value of 30 mmHg.

Example 4

In Example 4 the same compositions of the polymer solution and thecentre fluid were used as in example 1. The viscosity of the polymersolution was 73850 mPa×s The variation in polymer viscosity compared toExamples 1 to 3 is a result of slight variations between different lotsof raw materials within specified ranges. However, such variations arenormal and have to be accepted for the product grades which are used inthe membrane production processes. Nevertheless, for reproducibility ofthe process, the viscosities are measured to ensure that they are withinthe acceptable ranges of the present invention. The precipitation bathconsisted of 76 wt-% NMP and 24 wt-% water. The membrane formationprocedure was the same as in Example 1 with the exceptions that thespinning velocity was 10 m/min, the temperature of the die was 66° C.,and the temperature of the precipitation bath was 24° C.

The resulting hollow fibre membrane had an inner diameter of 257 μm, anouter diameter of 339 μm and a fully asymmetric membrane structure. Thetotal protein sieving coefficient was 100% at a transmembrane pressure(TMP) of 30 mmHg, 98% at a transmembrane pressure (TMP) of 70 mmHg and97% at a transmembrane pressure (TMP) of 110 and 150 mmHg (Mean Bloodflow Q_(B): 2.3 ml/min, mean shear rate: 190 1/s). The degree of freehaemoglobin as the corrected filtrate value (compare description of themethod) was below the border of starting haemolysis of 0.2 for thetested values of 30, 70, 110 and 150 mmHg.

Example 5

In Example 5 the same composition of the polymer solution was used as inexample 1. The viscosity of the polymer solution was 65500 mPa×s. Ascentre fluid a mixture of 34.0 wt.-% water and 66.0 wt.-% NMP was used.The precipitation bath consisted of 78 wt-% NMP and 22 wt-% water. Themembrane formation procedure was the same as in Example 1 with theexceptions that the spinning velocity was 10 m/min, the temperature ofthe die was 45° C., and the temperature of the precipitation bath was27° C.

The resulting hollow fibre membrane had an inner diameter of 323 μm, anouter diameter of 423 μm and a fully asymmetric membrane structure. Thetotal protein sieving coefficient was 100% at a transmembrane pressure(TMP) of 30 mmHg (Mean Blood flow Q_(B): 2.4 ml/min, mean shear rate:190 1/s). The degree of free haemoglobin as the corrected filtrate value(compare description of the method) was below the border of startinghaemolysis of 0.2 for the tested value of 30 mmHg.

Example 6

In Example 6 the same compositions of the polymer solution and thecentre fluid were used as in example 1. The viscosity of the polymersolution was 70700 mPa×s. The precipitation bath consisted of 76 wt-%NMP and 24 wt-% water. The membrane formation procedure was the same asin Example 1 with the exceptions that the spinning velocity was 10m/min, the temperature of the die was 64° C., and the temperature of theprecipitation bath was 30° C.

The resulting hollow fibre membrane had an inner diameter of 261 μm, anouter diameter of 343 μm and a fully asymmetric membrane structure. Thetotal protein sieving coefficient was 100% at a transmembrane pressure(TMP) of 30, 70, 110 and 150 mmHg (Mean Blood flow Q_(B): 2.5 ml/min,mean shear rate: 200 1/s). The degree of free haemoglobin as thecorrected filtrate value (compare description of the method) was belowthe border of starting haemolysis of 0.2 for the tested values of 30,70, 110 and 150 mmHg.

Complement activation was measured with minimodules in comparison to thePlasmaphan® and Cuprophan® membranes (Membrana, Germany). FIG. 3demonstrates the results. The TCC-values of the membrane produced inExample 6 are very low in comparison to the Plasmaphan® and Cuprophan®membranes.

Example 7

In Example 7 the same composition of the polymer solution was used as inexample 1. The viscosity of the polymer solution was 58900 mPa×s. Ascentre fluid a mixture of 36.0 wt.-% water and 64.0 wt.-% NMP was used.The precipitation bath consisted of 78 wt-% NMP and 22 wt-% water. Themembrane formation procedure was the same as in Example 1 with theexceptions that the distance between die and precipitation bath was 3cm, the temperature of the die was 60° C., and the temperature of theprecipitation bath was 30° C.

The resulting hollow fibre membrane had an inner diameter of 320 μm, anouter diameter of 418 μm and a fully asymmetric membrane structure. Thetotal protein sieving coefficient was 100% at a transmembrane pressure(TMP) of 30, 70 and 110 mmHg (Mean Blood flow Q_(B): 3.1 ml/min, meanshear rate: 250 1/s). The degree of free haemoglobin as the correctedfiltrate value (compare description of the method) was below the borderof starting haemolysis of 0.2 for the tested values of 30, 70 and 110mmHg.

Thrombogenicity measurements were carried out, and the produced membraneshowed excellent thrombogenicity properties (data not shown).

FIG. 1 shows a scanning electron micrograph of a cross section and ofthe inner and the outer surfaces of the membrane produced in Example 7.The inner wall of the membrane shows a very smooth surface on theselective separation layer. The outer wall of the membrane also shows avery smooth surface, but having large pores in the micrometer range.

1. A process for manufacturing an asymmetric hollow fibre membrane, comprising the steps of: extruding a polymer solution through an outer ring slit of a hollow fibre spinning nozzle and simultaneously extruding a centre fluid through an inner bore of the hollow fibre spinning nozzle into a precipitation bath, whereby the polymer solution contains 10 to 26 wt-% of polysulfone, polyethersulfone, or polyarylethersulfone, 8 to 15 wt-% polyvinylpyrrolidone, and 60 to 80 wt-% N-alkyl-2-pyrrolidone; the centre fluid contains 60 to 70 wt-% N-alkyl-2-pyrrolidone and 30 to 40 wt-% water; and the precipitation bath contains 70 to 82 wt-% N-alkyl-2-pyrrolidone and 18 to 30 wt-% water.
 2. The process of claim 1, wherein the polymer solution contains 15 to 21 wt-% of polysulfone, polyethersulfone, or polyarylethersulfone, 10 to 12.5 wt-% polyvinylpyrrolidone, and 66 to 76 wt-% N-alkyl-2-pyrrolidone.
 3. The process of claim 1, wherein the centre fluid contains 61 to 67 wt-% N-alkyl-2-pyrrolidone and 33 to 39 wt-% water.
 4. The process of claim 1, wherein the precipitation bath contains 73 to 79 wt-% N-alkyl-2-pyrrolidone and 21 to 27 wt-% water.
 5. The process of claim 1, wherein the N-alkyl-2-pyrrolidone in the polymer solution, in the centre fluid, and in the precipitation bath, if present, may be the same or different selected from the group consisting of N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-octyl-2-pyrrolidone, or mixtures thereof.
 6. The process of claim 5, wherein the N-alkyl-2-pyrrolidone in the polymer solution, in the centre fluid, and in the precipitation bath, if present, is the same and is N-methyl-2-pyrrolidone.
 7. The process of claim 1, wherein the polyvinylpyrrolidone in the polymer solution consists of a blend of at least two homo-polymers of polyvinylpyrrolidone, whereby one of the homopolymers of polyvinylpyrrolidone has a low molecular weight with an average relative molecular weight of about 10,000 g/mole to 100,000 g/mole, and another one of the homopolymers of polyvinylpyrrolidone has a high molecular weight with an average relative molecular weight of about 500,000 g/mole to 2,000,000 g/mole.
 8. The process of claim 7, wherein in the polymer solution, based on the total weight of the polymer solution, the low molecular weight homo-polymer of polyvinylpyrrolidone is present in an amount of 5.7 to 11.7 wt-% and the high molecular weight homo-polymer of polyvinylpyrrolidone is present in an amount of 2.3 to 4.3 wt-%.
 9. The process of claim 1, wherein the precipitation bath has a temperature in the range 10 to 60° C.
 10. The process of claim 1, wherein the hollow fibre spinning nozzle is held at a temperature in the range 40 to 75° C.
 11. The process of claim 1, wherein a distance between a discharge outlet of the hollow fibre spinning nozzle and a surface of the precipitation bath is in the range of 0.5 to 10 cm.
 12. The process of claim 1, wherein a spinning speed of the hollow fibre membrane is in the range of 1 to 20 m/min.
 13. The process of claim 1, wherein the polymer solution has a viscosity, measured at room temperature, of 30,000 to 100,000 mPa×s (Centipoise).
 14. A hollow fibre membrane obtainable by the process of claim
 1. 15. The hollow fibre membrane of claim 14, having a total plasma protein sieving coefficient of >0.90.
 16. The hollow fibre membrane of claim 14, having an inner diameter in the range of 100 to 500 μm.
 17. The hollow fibre membrane of claim 14, having a wall thickness in the range of 20 to 150 μm.
 18. The hollow fibre membrane of claim 14, having an average pore diameter of a selective separation layer in the membrane in the range of 0.1 to 1 μm.
 19. The hollow fibre membrane of claim 14, having a pore size distribution wherein the pore sizes at an inner wall surface of the membrane are smallest and said pore sizes increase towards an outer wall surface of the membrane where the pore size is biggest.
 20. (canceled) 